Hazards of heat exposure A review

DUKES-DOBOS FN. Hazards of heat exposure: A review. Scand j work environ health 7 (1981) 73-83. The usefulness of the various heat stress standards recommended by occupational health authorities in the United States is discussed from the point of view of their effectiveness in preventing acute heat illnesses. In this connection, the findings from an investigation of four recent fatal industrial heat casualties are described. A review is presented of the recent literature on acute occupational heat illnesses. Also reviewed is the information on chronic heat illnesses. Most of this latter data comes from studies performed in Europe and South America. However, a recent mortality study among steel workers in the United States contributed significantly to the knowledge of this problem area. This study found the primary targets of chronic heat illnesses to be the cardiovascular and the gastrointestinal systems, although there was some evidence that the reproductive functions may be affected. A discussion on how to prevent chronic heat illnesses is presented.

In common everyday speech the expressilon "heat stress" meanJS 1!hat a person is 'eX'posed ,to excessive heat and, as a consequence, hils or her healith is adversely affec'ted. 'I1hilS 'Colloquialism is a source of .confusion because in the scientific literatwre "hea't stress" is synonymous with "hea't load," which carries ftlhe connQltation that 'adv'erse health effects will occur only if the heat stf.ess exceeds .the perSOlIl's heat to'lerance capacity. At lower levels of heat stress there is no rliSk of hea1J1Jh damage, even though a pel'son may feel discomfort. ldea1il.y, it would be desi:rable Ibo eliminate heat stress completely by keeping the wor<kp}a'ce at a comfol1table temperature because the state of dilScomfort has many adverse :behavi!oral effects, suclJ. 'as reduced work rate (10), increased irritability, carelessness, and a feeling of fatrgue (21). These effects may render a worker more prone to aoccidentJal injul'ies (3,4). Un-for1Junawly, the cost of keeping all job sites at cOlIllTortable Itemperatures is prohilbiltive.
The aim of the heat stress standards recommended or promulgated in different countries is limited to the preventiO'Il of acute heat illnesses whkh result from exposure to heat stress beyond human tolerance. This is true also for two of the heat stress standards 'recommended in the United States, 'Ve, ror <tJhe one ir'ecommended in 1974 by the ad hoc Standards Advisory Committee ror Heai Stress of 1lhe Occupational Sarfetyand Health Administration (OSHA) (20) ;and l1he ifureshold limit value (TLV) of the American Conference of Governmenital Industdal Hygienists (ACGIH) (1), fkst puhlished iJn 1973. An exception is the heat stress standard recommended by the National Institute for Oocupational Safety and Health (NIOSH) (19), w:hi'ch limriJts ·the exposure time to heat in jOlbs wher,e unimpaired mental pemormance iJs critical from the point of 0355-3140/81/020073-11 view of safety. Unfortunately there are no data av.aiJlabloe at this time for determirning 1Jhe effectiveness of these exposure time lriimits in reducing !the number of acdderuts. To answer the quesmorn olf whether these two recommended standa'rds (19,20) and the TLV (1) would be effective in preventing acute illnesses, I have examined the prevai1iIng 'circumstances in four Tecent cases of 'fa1Jal heart strQke and one case of heat '€X'hlaustilorn whioh occurred in differem industries. From this analysis iJt is posSilble to draw some conclu$'ons aoout the use£u1ness of the preventive measures contained in the aforementioned documents.
The ipI'even'tion of chronic heat illnesses due to long-term work iJn hot environments has not been addI'eslsed <iJn tlhe recommended standards (19,20) and the TLV (I), because of Ithe SCal1'Ceness of dal1la on the health effects of exposures to low-level heat sbress over d'iIfferent per,iods of time. In this paper, however, fue pertirnent literature on the heaLth effects of ohronic heat illnesses ii'saJso reviewed, including a recent mortalilty study performed irn 'bhe steel industry (23).

Acute heat illnesses
'I1hecaooes of acUJte heat illnesses (table 1) are well established and have been described in detail 'by LeiJtheaJd & Lind (16). Wha't is still nOit Imlown for certain is why one worker sufifers from a heaJt cramp, anatlher collapses O!f heat exhaustion, while still another is able to continue to work in the same jolb as the other two wiJthout Name of heat illness and synonyms apparent ill effect, and then succumb to heat stroke when 'exposed. Ito hea't beyond toleI'ance levels. l!t can be <assumed that muclh depends on the worker's inherent and acquired physkal fiJtness and heat tolerance, as weE as on food and fluid intake. For instance, iii a person's cardio-vascula~capacilty is low, tiher'e is an increased probabiJity tiha't healt exhaUiSltion and oollapse may ensue before 'the body heats to the point at which heat stroke is triggered. This situation may actually benefit 1Jhat person <because one c,an easily recover from heat exhaustion if aHowed.
to rest m a cool pla'ceand drink cool liquids, while heat stroke is fatal if wholebody oooling (30) ils nOlt iootiated early in the iIlllCipient stage of \the di\selase. Similarly, a person may become incapacitated by muscle cramps due to incomplete replacement of the salt lost in sweat (17). This person, !however, will :recover soon after res1Jirng in a cool place and drinkilllg a glass of saline sohll1Jion, while andther, whose salt and water in.'take was saltisfa'01Jory, wi'll continue to work and later may become 1ihe victim of a more serious heat iJ1Il!ess.
On fue other hand, a mIld he,at illness, if not properly !treated, can lead rbo a more serious one. For instance, a periSon who col1apses from heat eX'haustion and is not taken out of the hot envi.rQIlJIIlent before becomi!ng ovenheated may develop a heatstroke as well. Similarly, saillt deplemon makes one more SUJSCep1lilb!le ibo eilther heat exhaustion or heat strdke (15). Furthermore, if workers become dehydr,ated beyond 1.5 Ofo of their ,total body weight,

Causal factors
Heat cramp Heat exhaustion, heat prostration, heat syncope Heat stroke, sun stroke

Skin diseases
Prickly heat} Heat rash Anhidrosis Heat edema Dehydration, hypohydration 74 Salt (NaCI) depletion, water and electrolyte imbalance Salt depletion with or without dehydration, resulting in hypovolemia and/or cardiovascular inSUfficiency Hyperthermia resulting in cellular damage in different organs of the body Reduced resistance of the skin due to constant wetting Obstruction of sweat g,land ducts by keratin debris; inflammatory reaction due to irritants or infection Excessive vasodilation in the skin Insufficient replacement of water lost by sweating due to drinking less fhlild than necess·ary for repl<aomg 'the walter IMt by sweating, not oniy willI their physical. and mental fitness deteriorate, but they also become more susceptible to acute healt illnesses and aoci'Clents (6). FinaJlly, dUJI"ilng work in a very hot and humid environment, the worker's skin will be oonstantly soaked by swealt. Thos'e who havesensrtiv,e s'kin or do not take a shower after work reguJarly or 'me i:n the tropiICs where 11he ruights are also hot and humid may develop heat rash, also called pr.i!dk1y heat. This conditioncauses the ducts of the sweat glands to becoane clogged, 'tihus !interfering with sweartmg. Workers sufreI'ling from this condition are highly susceptible to OIther a(:wte heat illnesses 'because they have lost their most effective mechanism for eliminating :he:art from their body, ie, evaporath'lllg sweat from the skin surface.
From the for,egoing preserutatioo irt; is app,l!re'llJt t'ha:t fif the workers in a 'holt plant o1iten suFfer mild iheat 'iilJ:nesses, J!lhe situation Should nm be diismissed as a harmless oondiJtilQll'l. Unfortunately, not every worker employed in hdt jobs nor Ittheir supervisors are aware of these acute heat illnesses, how to prevenlt them, how to recognize them, and how ito 'give first ·aro. Sometimes even physicians may have difficulty in differentiating between heat exhaustion and heat stroke in ,1Jhei.r early stages if the viotim'sbody rtemperature is rel,atively low (33). A'1tihough a rectal 1lemperatuTe of 40.6°C is considered to be the trigger point for an incipient heat stroke (11), a few 1eth'all heart stroke cases have been de-scr1i!bed in the 1itterature in whioh 1ihe patient's rectal temperature on admission to the hospdJtal was as low as 36.6°C (8). Of oourse, one cannot be sure how much time el'apsed 'between 1Jhe paltiienlt's lasrt heat exposure and the arriva[ at the hoopital and how much cooling was supplied rto 'the vii(ltiJm before tfhe rectal tempera.ture was measured. Recently, Bar1Jley (2) analyzed 24 cases of heart stroke which ocourred affiOlIlg soldiers of the US Army between 1972 and 1975. He came to the conclusion that the mulrtip'liJcilty of hosrt and env,ironmental factors whroh initeract in the causartion of heat stroke malke ilt doubtful whether total preventioncan ·ever be a1ltained under operational condiJtions of military-;braining, sports, and cerftain occ:upa.tions. Bartley based this statement on a S1tudy of heat stroke cases which occurred in tfhe US Army; thus it is in order to examine whe1Jher 'the generalization he made in his oonclusion, in pal'tiJcular the :field of occupational heat stress, is valid.
Ln 1977 and. 1978, NIOSH was' involved in explo!l":iJng the circumstances SU!I"roundaing fom cases of fatal occUipational heat stroke and one case of heart; exhaustion. In two of the heat stroke cases, NIOSH scientistsperiormed a field sUTVey a few weeks after the oasua,lity. They measured I11he envilronmenttal hea!t prev,aiIliinig 'at '1Jhe job Siltes where the heat oasuaJ.ties occurred and assessed the metabolic heat load o!f worl~ers who performed rfue same activities as ,fue heat stroke victims. In itJhe rem/aining ifuree cases, data on t1Jhe vdicHm's heat exposure a.nd other prevailing circumstances were obtained from OSHA inspectors.
The analysis at these daJta showed that <the :same environmental and hoot factors that ma:ke~t dilfficult iIJo prevent heat strokes in f1:he Army were also present in these industries. The most dangerous envkonmeIll1Jal fador is a sudden heat wave which finds the sdldiers, as well as 1Jhe wOl'lkers, unacd:iJmatized to this un-Ulsua1lly hilgh heat stress. However, fo[" industrial application the aforementioned recommended iheat stress standaJrds (19, 20) list some preveIllti've praetiJces whitch can be u5€ld in sii1luatioos !in which acclimatization procedures oannot be implemented.
:POI' iIlSitanoe, in iooustry it is possible to add some roUef workers to <the work force and increase ·tihe durart1i.ion or number of :rest periods; one (:an also reduce the heat load by protective clothing, mecharuiza:tion, automatiQill, and aV'a~lable climate-controlled rest p1Ja.ces. 'I1hUiS, as far as environmental heart; is concerned, oocupa-lriJe)ll.a!l heat oasualties seem rtx> be prevent-a1ble by proper a:dV'ance planning. As a matter of :flact, Bal'ltley himself sugges.ts later i.n his paper (2) that even in 1Jhe miJ1il1lary better planniillg fur ihot weather con'CllltiO'Il:S can sigrui.ficantly reduce the number and severiJty of heat casualties.
StilH, all J!lhese preventive measures do not address the problem of the hom faC'tors wlhich,aJocording l1lo Bartley 1(2), are par1l1y responsible for heat fatalities. To eliminate the susceptible individuals, the Army applies medical screening of recruits. Sim-Harly, the recommended heat stress standards (19, 20) contain requirements for preemployment and periodic medical examinations. Is is suggested in the mOSH criteria document for heat stress (19) that individuqJs who belong to the high risk group, such as the obese, the alcoholic, and the chronically ill, should not be placed in jobs with high heat stress. Furthermore, workers and supervisors should be informed about the conditions that make one more prone to heat illnesses such as dehydration, fatigue, lack of sleep, acute or chronic illnesses, and overeating. Table 2 Shows ,some of the host factors of Ithe five wonkers whose heat casualties wel'e aJna'lyzed. The first four cas'es relate to fatal heat strokes, and the fifth is a case of 'heat exhaustion wli:th coUaipSe. In three ouit of 'tJhe fiJv'e cases the wQll'kers were acclimatized to work in heat; however, sinoe ,a'll the oases occurred during a heat spen, one must assume that none of them were completely accllimatiz-ed to the prevaffiling heat stress on 1!he day they became ill. Ln :addition, each <Yf ,the five wonkens had one or more nega,tive condition in their medical history or in their physical fiJtness whiich may have reduced their heart; ltoleMnce. These data make it highily probable Ivhat host fadtJoirs played an impOrltant role iJn the causation of I!ili.e five heat Hlnesses. However, 1Jhe presence of oontI'ibultory 'host factors does no.t necessarily SUlpport 'the thesis of iJneviJtabiililty of aCUIte heaJt i'l:lnesses. Perhaps a more stringeIlJt medioal screening of workers being pla'Ced in hot jobs and closer medical moniJtol"ing of workers employed for many years in hot jobs could have prevented the occunrence of 1fu.ese -acUite heat illnesses.
As to the fatal outcome of the heat stroke cases, some fulIteresting faots were discovered. Ln i1Jhree cases the workers did not collapse on the j'ob. One was dressed to go home and collapsed near the exit, another collapsed on 1'tJhe way Ito 1ilie company's ipaJr~i!ng lot, and a ·third drove away in his car 'and collapsed at a neaJ.iby gas station. Aill three of the workers fe!Lt ill already hefore ·they left work, but nOlhody recognliz'€Id ,that they were in tJhe state of impending !heat stro:ke. One cannot but wander whether 'the lives of these workers c-ouM have been saved if the requirements of Ithe recommended heat stress standards (19, 20) had been adhered to; specifically in regar'd to the trnining of workers in hot plants on how to recognize acme heat illness anJd how to ghre mst aid.
Another si'grui:£i'CaInt finding was if1hat at the plants where the faltal heat casualties occurred, num,erous miJlder heat illnesses had been rt:rea'ted in the dispensaries during the same summer. Some of these heat illnesses were ill<l:t idenJtified as such, but were reconded in other disease cClJteg<lries. This raises the question, "How many heat iJlI1nesses go Ull.'reported?" El~1is (7) reviewed the oauses of excess deaths QCCurriJng in summer heat spellls and found a great iin<:rease in m<lrtality duri!ng these periods. He 'also observed that, in additiOlIl to a large linc,rease in fatal heat strokes, about ten 'times more people are listed as dying from diseases other than from !heat sbroke itse'lf. These diseases include arteriosclerotic and degenel'altive heart diseases, cerebrovarscular accidents, diJabetes, and genitourinary diseases, and they are probably aggrevated by the excessive environmenltal heat. However, death ce.I"1lificates rarely include info~on on conwiJbutory caJusal f,actors; thus the magnitude of the dealth to1!l taken !by the heat waves is much gl'ea!ter ttIh'an repopted offici-ally. Bartley (2) may be right, at least for the present, thalt aM heat stroke fatalities of occupational origin cannot be prevented. However, hased on the detailed analysis of the few !cases described in this report, the number od' oases could probably be greaJUy reduced wtth more stringent compliance to the preventive measures recommended in the standards (19,20).
In September 1979 I convened a workshop meeting under the aegis of NIOSH (proceedingsilIl pr,ess) Ito explore Whether recent accru'ed knowledge would make iJt possible rto modify 1fhe recommended heat stress staJndard (19) for the purpose of making compliance, as well as enforcement, simpler without reducing its efficacy. 'Dhe par'tioiJpanrtJs of 1:Jhds workshop recommended rt!hai the heat stress standard shoulIrl giv'e more fle:lribilLty to management in determining what preventive measures to use. However, they also recommended that preplacement and periodic medical examinations of workers in hot jobs should be mandatory. Furthermore, it was agreed that the standard should require that each industry which operates hot plants put in writing the policies and practices they adopt for preventing heat illnesses. This way the OSHA inspector could easily determine whether the policies and practices are acceptable and whether they are adhered to in practice.
A numbelr of problems could not be resolved aJt the workshop. Outstanding among 'tJhem was 1fhe level of hea,t stress above which a job must be considered a hot job Ifrom the poilllt of view of complia:nce requirements. The workshop paI-1licilpanrtlS ,stipulated howeV'er that a level IDentif.ied ,as the "action leV'€I" should be set below <the ACGIH TLV (1); it should be low enough to assure that none of the wOI1kers eJq>osed to these condmons will riJsk rbecomilJJga viotiJm of herat illness, not even unacclimatized workel'S. They also recommended r1fhat another level of heat stress should be set above rt'he ACGIH TLV (1) aJS a maximum permissilble exposure level. They recognized that, in order to establish these levels of heaJt exposure, more research is necessary, and a USit of research t"ecnmmendations was developed at the workshop.
One of the recommended research topics deals with a v~important aspecrt of the pl'eV'enmon of a'cute heat iUness,es, namely, to find the relat'ioI1lShi,p between deep-body temperature and the risk of heat illnesses. The guiding prilIlciple of the recommended heat stress standards, as well as the TLV (1,19,20), is that the workers' deep-body temperature shouJd Illot be permiJtoted :to exceed 38°C. '.Dhis prindple is based on the recommendation of a World Health OI1ganizaiion {WHO) sciellltJi.fic group report (31). Some pal'lticipants of the recent NIOSH workshop suggested 1fhat .this 38°C Hmit should not be interpreted as an absolute maximum permissible deep-body temperature. They f,elt rthat fluctuaJtions dudng the day may go up to 39°C without the individual beirng under undue stram as long as the 8-h time-weilghted average does not exceed 38°C. Scientists Clf the South African Ohamber of Mines base their per-miss~ble limits on aCCll'l'ate sta·tilStics on the number of 'heat strokes OCCUTring among miners. Stewart reported (Second International Mine Ventilation Congress, Reno, NV, 1979) that they acclimatized 350,000 gold miners in their climatic chambers annually in Souh Africa. This large worker population is very suited for epidemiologic studies. As a criterion for reducing the workshift time to 6 h, Wyndham et al (34) used 1:100 probability that the miners' rectal temperature reaches 38.6°C. FO'r a "SltOP-WOIik" limiit he used as a criterion t!he 1:2,000 proba:biliJty of the miners' r,ectal temperature reaching 39.2°C. Stewart (27) recen1Uy sugg'ested that the pel'IIlissible exposure limits for miners be based on a 1:1,000,000 risk that the fourth hourr recta[ temperature of a nude acclimatized miner will exceed 40°C. It is irnteresting t'O no'ue that recen'tlly he also estimated (Second Internaitiona:I Mine Ventilation Congress, Reno, NV, 1979) that the ACGIH TLV values correspond roughly with con-ditions carrying a 1:1,000,000 risk that unacclimatized men will reach a 40°C rectal temperature in the fourth hour of the workshift. However, 'a'ccording to Sltrydom (28), the SOUith Afdcan data CanlllQlt be adopted to US conditions because of differences in social, cultural, and nutritional factors. Goldman (9) observed that when the mean skin temperature of clothed men increased beyond 36°C, a rectal tem-95 """".. increase in red1Jarl tempemtu~e can raise the rirsk of heat ,collapse suhs,tantially. For nstance, if rectal temperatUJre reaches 39.5°C, the risk increases to 50 0/0. However, these data were obtained on young and physically fit military personnel; thus they also cannot be considered valid for industrrial workers in general. Indeed, the question of maximum permissible deepbody tempera,turecannot be decided until the research work suggested by the par-HdpaDJ1Js of the .Decent NIOSH wO'rkshop has been finished. Meanwhile, close adhel'ence to the recommended heat stress standards and/or TLV (1,19,20) remains the best way to prevemit acute heat illnesses. The ACGIH TLV and !the prev'entive work practices recommended in these standards are shown in fig 1 and  table 3. In cOnJtrast Ito ,acme hea,t illnesses, chronic heat illnesses have not been reviewed in the literature irn a systemaUc way. An att,emplt will he made to give a cOlIl!cise overview of relevant publications and b Only one mandatory if environmental and work intensity is monitored; more than one mandatory when no monitoring is done.

Acute illnesses
Chronic aftereffects Table 4. Chronic heat illnesses -Type I, aftereffects of acute heat illnesses. Table 5. Chronic heat illnesses -Type II, cumulative effects of long-term exposure.  (table 6). Chronic, type I heat illnesses (table 4) may not be diagnosed as clear-cut disease conditions; however, it has been well established that a person who has once suffered an acute heat illness (except heat cramp) has a reduced heat tolerance for some time thereafter, perhaps for the rest of his/her life (16). After a heat rash this phenomenon may be due only to reduced sweating capacity; however, after a heat stroke it may be the consequence of irreversible cell damage in the brain, liver, kidneys, and other organs of the body (33). The severity of the cellular damage largely depends on the severity of the heat stroke, particularly on the length of hyperthermia and on the maximum deep-body temperature reached.
Chronte, 'type II heat il'lnesses (table 5) are the resUilt of long-term exposure to work in heart;. It iJs ndt quirte clear whether the symptoms observed clinically and the aJbnormalities fOUiIld by medicaUy screening wOI'ker popu!l.ationsare part of one and the same heat ililness or whether different chronic heat illnesses may develop depending on the severity and iength of exposure. Alternatively, it lis possiibJle that the same healt stress may trigger different illnesses in different persons and may depend on the constiitut~onal fa·ctors and the medical history of the inrliviJdual. Schwartz (26)Qlbserved <an illness 'among workern in hdt jobs which occurred several months after theitr employment. Thilis illness was charac'teriz'ed by 'the set of symptoms shown in tab1e 5. These woTikers were accl.i!maroized 1Jo healt but were not accustomed to the heavy physica'l demands of 'the job;lJhey were ambitioUIS individuals who wou'Ld not girve U!p \their jobs in spite of 'the strain 'they were experrencing. As a consequence, I1!heir symptoms gradwal,ly became worse, and, if 'these workers were in the state of chronic 'fatigu'e without heat exposure. However, a general feeling of weakness, irdtability, vertigo, and nausea has been observed (5) in the Hungarian steel industry during the hottest parts of the summer,31IIlong the workers whose sweai 'rate reached 7 to 9 l/8-h workshift during a heat spell lasting 5-10 d. The workers' urinary chlorine concentration before and dUI1mg the workshift was practi-caLly~ero during Ithis period andllherefore Showed the extreme effort exeI1ted by their electrolyte regulating mechanism. The symptoms descr~bed by Schwartz (26) thus could !have been, a1 least partially, due to a chronic form of lS'a'lt depletion heat exhaushoo.
Apparently, ltIhe level of heat stress to whkh these workers must have been exposed is below the heat stress whioh would cause an acute heat ilJLness, bUit it is beyund the 'leV'e'l oaned the "chronic heat tolerance limi1" '(CHTL). This liJmiJt was fOUiIld (5) to be, in terms of sweat rate, 5 1/8-h workshiit for the average acclimatized worker. It is based on Ithe assumption tfuat the daily salt intake of a worker is abault 5-6 g during the workshiflt 'and 7-8 g during the rest of the day. Wlhether the CHTL can be increa,sed rto IhiJgher levels, if the sallt intalke is increased during the workshift, is one of the questians which cannot be answered wi,thoult further research. It has been postulated that illlicreased salt iJnitaike may cause potassium depletion fram the musde ceills and, thus reduc,e muscuJ.a'l" strength and physical rperformance capaciity. (28).
Another poolJlem of increased salt intake is related Ito its effect on blood pressure. K:loietzel et al (13) screened the 350 employees 'Of a Brazillian Slteel plant for blood pressure. He fuuJlld that the proportion of mild and severe hypertensives (above 18.67/ 21.33 and above 21.33/12.67 kPa) was signicanitiy higher ,among the 186 workers of the blast furnace, open hearth, and rolling mills operations who were exposed regularly to high heat stress. The prevalence af severe hypertension was greatest (30 0J0) among the workers employed in ibhese jobs for as 'lOiIlsgas 20 ;to 29 'a, whereas among those employed from 0 to 4 a, the prevalence was only about 7 0J0. This rela-rtlionshiJp Ibetween an increase in prevalence with time \Spent on the job occurred in spite of ;the fact ,that many workers were laid off at an earlier age because of hypertensive cardiovascular disease. Therefore few of ,the workers reached the legal reti!remen1t span of 35 a of work. Kloetzel et al (13) considered ttihese findings tentative because they :£elt that a better controlled study woul1d be necessary to con-JfiJrm ;tihese preliminary findings. However, they 'Presented evidence Jthat the observed e:x:cess of hypertension among the workers e:x:pQsed'to inten'se heat was not due to an uneven age or racial composition of the cohort or to eXCesSive salt intake. As a matter of fact, even though salt dispensers were avaiilalile in the plant, the workers avoided them Ibecause of 1Jhe rumor that the salt tablets might be responsible for impotency.
'Ilhis :latter abs'ervation brings i!nio focus two major health problems conneoted with long-term employment in hot joibs. One health 'Problem iJs related to the fact that hypertenSive patients must be kept on a low sodium diet which requires low salt intake. This si!1Juation confronts rtJh.e hypertensive workers of hot plants with a dilemma: iJf they reduce thei'r salt intake, they may become more susceptiJlile to acute or chronic heat exhaustion and heat cramps; 'if they maintain or -increase their sa1tillltal ke, :1!hey may aggravate their hypertension. IIll addition, if workers of a hot plant aTe tr,eated with diuretics -the medication of choice for certain types of hypertensiJOn -'iJhey wiJ1l be ,a:t a disadvantageas far as healt tolerance is concerned because t'heyare allready slightly dehydrated even before ,they start to swealf. As a matter of fact, exper1ments performed on spontaneously Ihyperten.si!ve r,ats in our laboratories by Wright et al (32) showed thalt hypeI'tension per se may reduce heat tolerance sig!Illi.fiJcantly.
This hearllth problem has already been recognized by some Physidans who became ,aware of 1Jhe faet that severa'l of their hypertensive patients are e:x:posed to m-1Jense heat on !the joib. They try to compensate ,for this fact by pres'criibirng a lower dose of diuI'€Itios during the summer. However, '1lhJis is hardly a satisfaotory solution to ,the problem. It 'has to~eslla'blished by systema.uk reseaI"dh how much heart; tolerance is lost art dilfferent stages of hyperten,sion and w~1Jh diJfferent regimens of treatment. Fur'1Jhermol'e, it !has to be determined e:x:a'etly how mueh heat sltress, over how long a period of time, wiilI make workers more susceptilble to hyper,tensive caooiov:a:scular disease. The va~ue of the CHTL has to be set according to eo level Whiidh .i:s serle for hypertensive indi'Viduals. The only oiher alternatlive woulld be to exclude hypertensive workers from hot jobs by ,preemployment and periodic medical eXiaJI'Ilinations.
The other health problem referred to in 1lhe paper of Kloatzel et al (13) is Itfhe fact that 1lhe workers i!n !hlQt jobs are concerned about thei!r sexual potency. A·]though these W1OI1kers !blamed the &allt tablets for thek impotency, it iJs more probable thaJt the heat stress to w'hkh they are exposed is culpable for their sexual defidency. I hav,e often hea'rd complaints of reduced libido when Ital1kilng to workers in hot shops, but is was not dear whi'ch of the many stress factors present in thes,e jobs should be suspected of being responsible for this phenomenon. However, Knecht et all (14) recen!1ily ex;amined, i!n our laboratory, the effeot of 'heat stress on the reproiductiv'e functions of male and female ralts. They observed 1Jha;t daily exposure of the male rat far 55 min at 38.2°C decreased copulation and reduced the rane of concepbon. The same ,exposure of females caused diJSruption of 'their estrous cycle until they became .acclimatized to heat. Both male and f,ema:le exposure to this shol't bUit imten.se heat on a daily basis adversely affected 1lhe ,survival of the fetus. During laotatilQnthe females' heat tolerance decreased siJgmJiificantly and resull:!led in increased mortaJ.iJty due to heat stroke. The resuHs of these mimal experiments cannot be direotly extrapoilaited to man; however, it iJS quite possible ,tha,t fUiture research on human sU!bjeots win reveal 1!hat the r'eproductiv,e funotions are very sensItive to heat stress and therefore should play an important role in deiter-mining the level of ,the CHTL.
OtJher ohronic health effects observed among workers employed in 'hot jobs (table 5) suggest that just as in the staJte of chronic heat exhaustion which develops after a few months on the job (26), it is the cardiovascuilar 'system which is the most M-£ected by many years of employment in hot jobs. Kenedi (12) found pathological electrocardiograms four times as frequently among Hungarian foundry workers exposed to intense heat than among those ex-posed to moderate heat. Mayer-Theveniaud (18) found a higher incidence of cardiac conduction anomalies among French workers in hot jobs. At the 7th Congress of the International Ergonomics Association (Warsaw, Poland, 1979) Kiec reported that the 217 foundry workers who perform the hot!test jobs in the Lenin Steel Works had the worst hemodynamic parameters as determined Iby polycardiography. He emphasized thalt tJhis group of workers was highly self-selected, physically the most filt, strongest and healthiest in vioew of other clinical parameters. He postul,ated that the deterioration of hemodynamic parameters was due ,to myocardial damage sustained during >the short periods of heavy work under extremely high heait exposures, When the heart rates o.f the workers reaohed the,ir maximUlm. If further r,esearch confirms this pos,tulare, lot should have ,significant impact on what exposure liJmiJts to recommend in a heait stress standard. At present, action levels and exposure limlits in both of the recommended stmdards (19, 20) and the ACGIH TLV (1) are expressed in terms of tim,e-weighted averages. If KieC's CO'Ilclusions are verilfied by future resea-rch, then it becomes imperative to include limiJting levels for peak exposures in the heat s-tr·ess standards, even if such exposures occur only for a short peIiiod, 1 or 2 min, each workshift.
There i,s some evidence that, in addi,tion to the cardiovascular system, the digestive organs aflealso affected by 10ng4erm exposure to heat. Redmond e't al (23) performed a mortality study under a NIOSH contra-ct, for w1hiJch I served as project director. The study cohort cOiIlSisted of steel wQflkers emp10yed in large foundries located in Allegheny County, PA. The only diseases more frequent ,than cardiovascular disease as the cause of death among workers exposed to intense heat wefle ',the nonmal1gnant dilgestive diseases. One possible expl,anation. for this finding is that phy:si'OlogicllIl adaptation to healt involves peripheral vasodilatiiOn and a compensatory v.asoconstriclion in the splanchnic diJgesti'Ve organs. When this condition prevails day-ajJter-day for many years, it can cause a reduced funct.iJonal capacitty and a lowered resistanoe to disea·se. Indeed, Rowell et al (25) found evidence of hep&tic-splanchnic hypoxia in subjec;1;s exposed to 'heavy exercise at an environ-mental temperature of 48.9°C. Experiments performed in our 'laboratories by 'I'm'asson et al (29) showed a decrease of mucosal tissue and a decreased rate of glucose m~tabolism in the initestines of heat-e}Cposed rats. FUl"thermore, in a personal communicatiJon, FM Spiloch reported a decl'ease in the level of hemoglobin among workers exposed to intense heat in some Polish steel plants. Although there can he several possible causes for this disorder, at least !thl'ee of them are connected wirth disorders of the digestive organs: occullit bleeding from the intestines, atrophy of the gas'triJc mucosa, and hyporia of the liver.
Mor-e research is needed to determine the intensity and length of heat exposure which wiH result in gastrointestinal disol'ders. However, Redmond's study (24) presents good informa,tion about ,the yearround environmen:tal and metabolic heart load of 11he jobs in whi-ch the members of the -cohort were employed. It is interesttilng to note that a significantly hi:gh risk for digesti'Ve diJseases (excluding cirrhosis of the liver) was -round among persons who had ever worked in jobs above the ACGIH TLV (1). In addition, those who worked in Jobs below the TLV for 15 a or longer also had iJncreased mortality due to digestive diseases. 'Dhese findings suggest that ifue values of a CHTL mU'SIt be set lower than the current TLV irf disorders of the digestive organs are 'to be prevented among workers -employed in hot jobs for 15 a or more.
It is ,interestim,g to note that the study cohort of Redmood et al (24) did not show an increased mor.tality due Ito cardiovascular diseases, except for those workers employed less -1Jhan six months in hot jobs. This fmdiJng suggests that, by way of nat.. urarl selection, workers with low cardiovascm'laT fiitness do nat stay longer than six mon1Jhs in the 'hot jobs of Ithese steel plants. Those 'r,emainiJng in the h(jt jobs for more tharn 'six monlths apparently have a be'1Jter than a'Verage cardiovascul'ar fil1mess, and, in fad, their mortality due 'to cardiovascular diseases was less than that <>if lthe reference grOOJp. Table 6 shows the symptoms of chronic heat illnesses whJJch have been observed among residents of cliimatically hOltg ioIl!S. A concise 'review of disorders observed in people who moved from tem-82 pel'ate regions ,to tropical zones has been pI1esen:ted thy Pepler (22). He 'leaves the question open 'of whether heat stress in the hot and humid tropics causes different symptoms :than when the exposure to similar environments iJs 'limited 1IJ0 the workplace. He suggests that 'cultural, sodal, and hygiJeni1c faotors may play a greater roll in tropkal neurasthenia than climatlic stress. hccordingly, !its prevenJtion is best a'chieved by psychoLogical methods, although iIJ!trodudtion of air conditioning can go a long way. Anh!iidrOltiJC heat exhaustion, as descriJbed by Leilvhead & LiJnd (16), seems to be a condition limi<ted to the milHary personnel stationed riD. e:itlJher tropkal-or desert-type climatic regions. It usually occurs af:ter about a four to seven months' stay in these hot locations. However it is not quilte dear Whether it is a separate disease entity or just a chronic and more severe form of prickly heat, occasionally combined w~th chronic heat exhaustion.
A high incidence of kidney stones among residents of desert regions has been men-tiIO'noo by several iJnvestigat:ors (14,15) and is 'considered to be a cOlliSequence of adaptation to inadequate water consumption, which results in highly concentrated urine. Workers iJn 'tJhe desert regions do not realize !how much lt1hey ar,e swealting because the sweat evapora'tes immedioately from their skin due to the dryness of the air. They ShouJ1d ,be encouraged to drdnk more water than l1Jhey would spontaneously.